Configurable network using dual system busses with common protocol compatible for store-through and non-store-through cache memories

ABSTRACT

A network of digital modules having store-through and non-store-through cache memories, is provided with intercommunication capability by means of two sets of system busses each of which are replicates of each other. The system busses provide a higher throughput by both being available to each of the digital modules so that a requesting digital module can alternately use a second system bus if the first system bus happens to be busy. Failure of one system bus will allocate transmission service to the second operating system bus thus providing redundancy. Alternatively, each of the system busses can be isolated for partitioning the digital modules into two different operating systems which are independent of each other.

This is a continuation of co-pending application Ser. No. 08/019,003 filed on Feb. 18, 1993.

FIELD OF THE INVENTION

This disclosure relates to a digital module network provided with dual system busses which are duplicates of each other and have particularized sets of lines for specialized signal data and wherein the dual busses provide redundancy and partitionability.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is related to the following co-pending applications and U.S. patents on related systems and circuitry which are included herein by reference.

U.S. Ser. No. 07/961,744 filed Oct. 16, 1992 on "Programmable Timing Logic System for Dual Bus Interface"which was allowed in June 1995.

U.S. Ser. No. 07/837,174 filed Feb. 14, 1992 on "translator system for message transfers between digital units operating on different message protocols and different clock rates", now U.S. Pat. No. 5,444,860.

U.S. Pat. No. 5,404,462 entitled "Transfer System for Central Processing Module."

U.S. Ser. No. 08/018,996 entitled "Dual Bus System for Store-Through and Non-Store-Through Cache Memories in Multi-Module Network Involving Sending and Receiving Modules."

BACKGROUND OF THE INVENTION

As the networking of multiple types of processors, input/output units and peripherals increases in scope and complexity, many unique problems arise involving the intercommunication of commands, messages, and data between the variously interconnected modular units. It is necessary that there be taken into account means for allowing flexibility within the system, means for accommodating different types of processing units, and means for allowing partitioning, if required, all in addition to providing high system speed and throughput with proper data transfer, integrity, and error correction means. Further, networks which use only one system bus are subject to halt-down should the system bus fail so that a dual replicatable system bus can be used to eliminate this problem to provide redundancy. Such duplicatable or replicatable system busses incur some additional problems in regard to communications between the various modules.

Prior art systems such as U.S. Pat. No. 4,622,630 entitled "Data Processing System Having Unique Bus Control Protocol" involve a single common bus which is used for communication of address and data information among various system components but wherein the single bus is time multiplexed in order to provide periods for address transfer and periods for data information transfer thus slowing the throughput to some extent.

In U.S. Pat. No. 4,982,321 entitled "Dual Bus System", there is provided an architecture where two system busses are used for entirely different purposes and are not replicates of each other. Thus one bus in this system is operable for memory operations while another bus is operable for input/output operations, but there is no redundancy or means by which any one of the busses can be used for all of the necessary functions. Thus this type of system could not operate on a single bus alone since it, at all times, requires at least two busses for normal operation.

U.S. Pat. No. 4,535,448 entitled "Dual Bus Communication System" involves a dual set of busses used to provide coupling between data services and voice services of a particular AT&T communication system designated as the CS 300. In this system, one bus functions on a time division basis for communication between port circuits, while the other bus functions as a packet-switch data processing bus for interfacing system peripherals with port circuitry. Again here the busses in the system are dissimilar and are not interchangeable and thus cannot provide any redundancy.

In U.S. Pat. No. 4,933,846 entitled "Network Communications Adaptor With Dual Interleaved Memory Banks Serving Multiple Processors", the system involves dissimilar busses such that one bus functions for transferring addresses and the other functions only for the transfer of data.

The architecture of the presently disclosed system network involves particularized dual system busses which are replicates of each other but are also capable of operating independently and thus providing redundant bus transfer facilities to each of the attached digital modules. Thus each one of the two system busses involved is a duplicate of the other so that the system can operate on any one of the busses should either system bus fail.

Further the dual system busses operate on a single type of protocol which is compatible for processors having Store-Through (ST) cache memory units and also for Non-Store-Through (NST) cache memory units.

Thus the objects of the invention are to provide an enhanced throughput of data transfer operations between multiple types of processor modules, multiple memory modules, and multiple input/output interface modules and permitting, not only redundancy, but partitioning and flexibility of bus arbitration and flexibility of system operation while using a common bus protocol applicable for use of store-through and non-store through cache memories.

SUMMARY OF THE INVENTION

The presently described network provides a pair of specialized dual system busses enabling capability of intercommunication with different types of digital modules which include processor modules, memory modules, and interface modules. Each one of the specialized dual system busses provides separate sets of data lines for different functions such as the address/command/data transfer function, the control function, the status information function, the arbitration function and clock signal transmission.

Multiple choice of bus pathways are provided for each module when it functions as a module requesting bus access for communication to a target module. Multiple pathways are provided via the dual system busses also for receipt of data when a module becomes a target-receiving module. The network with the dual system busses provides for simultaneous operations wherein a single module can perform the functions of both transmitting information on one bus line and receiving information on another bus line thus enhancing the throughput.

The dual system bus network provide an equitable fairness arbitration scheme to insure that no module is completely eliminated from bus access while at the same time providing priority to the most needed access by certain modules. A snooping or spy logic circuit in each of the modules permits not only error detection and data transmitted on the bus, but also resolves priority of function when two simultaneously requesting modules seek the same area of memory.

Partitioning of the network system, by a Maintenance Processor, is possible by allocating each one of the system busses to a different set of digital modules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system network wherein dual system busses are connected to each and every one of the active modules which may include the central processing module, the memory modules, and other modules such as the interbus module and arbiter modules;

FIG. 2 is a drawing showing the details of one of the system busses and how the various functions provided are apportioned on individual lines of the bus;

FIG. 3 is a schematic drawing of a typical central processing module which indicates various sections designated as the processor logic, the transfer box logic and the bus interface unit logic;

FIG. 4A is a architectural drawing showing how the dual system busses are interconnected with a number of processors which use Store-Through cache memory units;

FIG. 4B is a network diagram showing how the dual system busses are connected to processors having a Non-Store-Through cache memory unit.

DESCRIPTION OF PREFERRED EMBODIMENT

The dual system bus and digital module complex described herein is incorporated in a multiple processing system complex (FIG. 1) developed by the Unisys Corporation of Blue Bell, Pa. and designated as the A-11 computer system. The system busses 22A, 22B, connect various processing elements (such as central processing modules 8a, 8b, and input/output units 68a, 68b) and memory modules 48 to provide a shared communication path between these modules. In FIG. 1 each of the connected modules will be seen to have a Maintenance Interface (MI) Unit for maintenance purposes.

Since the A-11 computer system will support different types of processors, it may be illustrative to indicate such types of processors. For example, the processor 28 of FIG. 3 may be implemented in different versions of processing architecture. One type may be called the IMP processor (FIG. 4A) 28I and which processor has a cache unit which operates as a Store-Through (ST) cache memory unit. Another version of the processor 28 of FIG. 3 may be a processor designated as the CHAMP processor, 28C, FIG. 4B, which processor has a different type of cache memory unit designated as a Non-Store-Through (NST) cache memory unit. The dual bus protocol will support both the Store-Through cache operations and the Non-Store-Through cache operations so that the bus has the flexibility of working with two different types of processors having cache memory units of a different nature.

In a Store-Through (ST) environment FIG. 4A, the dual bus capability will allow processing elements (PE) to access memory modules only. In a Non-Store-Through (NST) environment, the dual bus capability allows processing elements to access both the memory modules involved and private NST cache memories of the processor modules. In addition to servicing both types of environments, the dual bus capability will support message transfer and other status access information such as acquiring time-of-day information (TOD) between processing elements and other modules.

The system bus capability is of a dual nature so that one system bus 22A is a duplicate or replicate of the other system bus 22B permitting the system operations to avoid single points of failure, to isolate system performance when a partitioned system is desired, and also to increase the throughput and overall system performance of joint systems. Thus the two system busses 22A and 22B are essentially identical and referred to as System Bus A and System Bus B, seen in FIG. 1 as 22A and 22B.

Each digital module on the system busses connects to both of the busses A and B. In a dual or "joined" system configuration, as set up by Maintenance Processor 40_(p) of FIG. 1, the module on the bus can use either of the two busses as availability permits. If one of the busses fails to operate, the system can still continue to operate with only one bus being operational. The broken bus is deactivated by the disabling of "bus grant commands" to the requestor modules on the failed bus.

In a split or "partitioned" system, as set up Maintenance processor 40_(p) of FIG. 1, a particular bus is assigned to each partition and the modules in the partition can only use the bus assigned to that partition. If the particular bus assigned to a partition should fail, the failed partition would be taken off-line.

GENERAL OVERVIEW

The system busses provide a shared communication path between processors such as processors 28A and 28B (FIG. 1), in addition to the main memory modules 48_(abcd) and the input/output modules designated as the interbus modules PMIU 68_(a) and 68_(b). The system bus permits modules to access main memory and cache memory units, to transfer messages and to access other states of information such as a time-of-day (TOD) counter in the interbus modules 68a and 68b.

In order to avoid single points of failure, the bus is duplicated and thus permits isolation of system performance into a partitioned system and also can enable and improve performance of a joined system. Since the two system busses are basically identical and since all the modules in the system connect to both of the busses, various flexibility in operations are permissible. In a "joined" configuration, the module on the bus can use either of the busses as bus availability permits. In a partitioned "split system", a particular bus is assigned to each partition and the modules in that partition can only use the bus assigned to that particular partition.

In the presently described configuration, the system bus may be run synchronously on a 16 megahertz clock, for example, shown as item 19 in FIG. 2. As seen in FIG. 2, each system bus is divided into five segments for handling transfer of specialized data such that the bus segment 22_(d) will handle address/and command/data while segment 22_(c) will handle control data, segment 22_(s) will handle status information, segment 22_(a) will handle arbitration operations and segment 22_(k) will provide the clocks for synchronizing each of the module activities.

The system bus operates as a synchronous bus and the command set for the bus is so defined as to support both Store-Through (ST) and also Non-Store-Through (NST) cache memory environments. The access of a requestor module to the bus is handled by bus arbitration units 18_(a) and 18_(b) shown in FIG. 2. In another configuration such as indicated in FIG. 3, the system arbitration may be provided by a system arbitration circuit module within the central processing module 8 of FIG. 3. Thus bus arbitration can also be handled through a central arbiter being retained in the central processing module 8 at just that position to the Central Processing Modules 8_(a) and 8_(b) (FIG. 1). The bus arbiter such as 18_(a) and 18_(b) of FIGS. 1 and 2, implements a "fairness" arrangement in order to assign bus availability to the requesting modules. It only requires a single bus transaction to complete the request for data access and the data return execution. The address signals and the data signals on the bus are multiplexed and are checked for integrity with a byte parity arrangement.

SYSTEM BUS PERFORMANCE

The time-for-access of data access operations will depend upon the memory response time in the memory modules 48. The system bus is designed to accommodate different types of memory modules having varying performance. However, the minimum number of cycles in a bus transaction will be seen as indicated in the Table I hereinunder.

                  TABLE I     ______________________________________     SYSTEM BUS PERFORMANCE     COMMAND   CYCLES        ACTIONS     ______________________________________     Read Word 3+ memory access                             arbitration+command/                             address+memory access                             +data return     Write Word               3             arbitration+ command/     Write SP,               address+write data     (Single     Precision     word)     Read Lock     Write     4+ memory access                             arbitration+ command/     Protect                 address+write data+                             memory access+data                             return     Private   6+ memory access                             arbitration+ command/     Read, Shared            address+memory access     Read                    +4 word data return     Purge Block               7             arbitration+command/     New Data                address+4 write data     Write                   cycles     Send Message               7             arbitration+message                             cmd/destination ID+4                             write data cycles     Read TOD  3+ TOD access arbitration+message     (time of                cmd/destination ID+TOD     Day)                    access + TOD return     ______________________________________

BUS CHARACTERISTIC SUMMARY

The following summary of the system bus is indicated below in items (a) thru (m):

(a) The bus connects two central processing units having the IMP processor 28I (28I_(a), 28I_(b), FIG. 4A) or alternatively two central processing modules 8 having the CHAMP processor 28C (28C_(a), 28C_(b), FIG. 4B) which uses the Non-Store-Through (NST) cache memory unit, plus two interface modules (PMIU) designated 68_(a) and 68_(b) plus four memory modules 48_(a) through 48_(d). (FIG. 1).

(b) The system bus allows the IMP processor module (28I) to access memory on a "word" basis. Alternatively it permits the CHAMP processor module 28C and also the interface modules 68_(a) and 68_(b) (PMIU) to access memory on a 4-word block basis. The acronym PMIU refers to a Processor-Message Interface Unit, which has been given the shortened designation of an interbus interface module.

(c) The bus supports 4-word message transfers and also (TOD) time-of-day access between the IMP processor (or alternatively the CHAMP processor) modules and the interface modules 68_(a) and 68_(b) (PMIU).

(d) The bus (excluding power and ground) can carry a total of 98 signals which are divided into four groups as follows:

(i) arbitration signals;

(ii) command, address, and data transfer signals;

(iii) control/status signals;

(iv) miscellaneous signals such as the clock signal and reset signal.

(e) The system bus carries address signals and data signals which are multiplexed.

(f) The system bus provides for a data request and a data return in one signal bus transaction.

(g) The system bus provides for a central or a distributed arbitration system enabling a fairness scheme so as not to starve out any particular module. The arbitration is performed on every clock and concurrently with any current bus transaction. Each arbitration cycle will establish one bus transaction.

(h) Each "bus transaction" on the system bus can have one of the following requests (or transmit) phases:

(i) one command/address phase;

(ii) one command/address phase followed by one data phase;

(iii) one command/address phase followed by four data phases;

(iv) the data return (receive) phase will follow the request (transmit) phase if the requesting phase is expecting a data return.

(i) The command/address bus phase is always within one clock cycle. The data phases are executed within the minimum of one clock cycle but can be stretched to multiple clock cycles by asserting a signal called the Not Ready signal designated "NRDY".

(j) The system bus provides byte parity protection for various fields such as the command field, the address field, and the data field.

(k) The bus provides support for the transfer and identification of "corrupt" data (data which has errors).

(l) The bus supports a "retry" feature which is supported with a retry waiting period and a "retry limit" for error conditions and resolution of cache coherency.

(m) The system bus functions to provide a "time-out" limit on a bus busy signal designated as BBSY; it also provides for a signal designated Not Ready (NRDY) when a designated target module is not able to receive or transmit at a given moment in time.

BUS COMMUNICATION PROTOCOL

The system bus 22 provides a shared communication path to modules through a defined set of signals used for arbitration, for command/address and data transfer and for control/status information. The bi-directional signals for command/address and data transfer are multiplexed.

MODULE TYPES

As seen in FIG. 1, the system bus 22 (22A, 22B) connects a total of eight modules. These include central processing module 8_(a) having processor 28_(A) and central processing module 8_(b) having processor 28_(B), plus two interbus memory interface modules (PMIU) 68_(a) and 68_(b), and four memory modules 48_(a), 48_(b), 48_(c), and 48_(d). The system bus 22 can support both types of cache environments such as the Store-Through (ST) and also the Non-Store-Through (NST) cache environments. These two types of environments are set up and configured by the Maintenance Processor 40_(p) of FIG. 1. In both these type of environments, the memory modules 48 act as a server module while the interbus module (PMIU) 68_(a) and 68_(b) act as a "requestor" module on the bus. The processor modules 28A_(a) and 28B, however, can act as a requestor module in a Store-Through (ST) environment (FIG. 4A). In a Non-Store-Through (NST) environment (FIG. 4B), due to private Non-Store-Through (NST) caches, the processor modules and the requestor and (responding module) act as requestor modules. The requestor modules make use of the bus to access data and to transfer messages to each other. The "server modules" use the bus to transfer requested information to requestor modules.

When a module is requesting information to be returned to it, then it is called a "requestor module". The "server modules", such as the memories 48, use the bus in order to transfer and give out requested information to requestor modules seeking such information.

COMMAND TYPES

There have been defined a set of commands for operation on the bus which are applicable to both the Store-Through (ST) and also the Non-Store-Through (NST) environments, thus allowing modules to communicate with each other. The "bus command set" can be categorized into two basic types:

(i) a memory type command and

(ii) a message type command.

In a Store-Through (ST) environment, the memory type commands are targeted to memory modules and data is always accessed from the main memory either on a word basis or on a 4-word block basis. Some of the "Memory Write" type commands also cause ST (Store-Through) cache invalidations.

In a Non-Store-Through (NST) environment, the "memory type" commands are targeted to both memory modules and to the Non-Store-Through cache units on the processor modules. Data is accessed from either of the modules on a 4-word block basis only. Some of the "Memory Read" type commands will cause Non-Store-Through (NST) cache units to return data. Write type commands may cause cache unit invalidations to occur.

BUS PHASES

Communications on the system bus involves the following bus phases:

(a) Arbitration Phase;

(b) Transmit Phase;

(c) Receive Phase.

A "transaction on the bus" which depends on the bus command, is made up of the Arbitration Phase followed by either a Transmit Phase only, or both a Transmit and a Receive Phase. Table I indicated a set of commands and listed the number of cycles involved in the Transmit and Receive Phases associated with each command.

(a) Arbitration Phase: A transaction on the bus is established first through a bus arbitration phase. The bus arbiter 18 (FIGS. 1, 2) determines the next master control for the bus, based upon current requests, the priority of requestors and previous bus grants. This phase of the bus transaction is overlapped with the end of either a transmit phase or a receive phase depending on the previous command executed. The bus arbiter 18 arbitrates on every clock and can issue a "bus grant" on each bus to two different requestor modules, but the module receiving the grant does not drive the bus until the bus is available for the start of the next bus cycle.

In the A-11 computer system, the bus arbiter 18 may be in the central processing module such, as 8_(a) and 8_(b), and FIG. 3, or it may be an independent module connected to the system busses as seen in FIG. 1. Several different configurations may be operative. In a "joined" system configuration only one bus arbiter 18 is enabled. However, in a "split" (partitioned) system, both arbiter modules, such as 18_(a) and 18_(b) are active. The bus arbiter 18 maintains a 2-bit state relative to each bus request line. The state indicates if a particular requestor module is allowed to use system bus A, 22A only, or system bus B, 22B only, or both the busses or none of the busses.

Table II shows the bus arbiter state values for enabling each of the busses.

                  TABLE II     ______________________________________     BUS ARBITERS STATE VALUES FOR ARBITRATION     REQUESTS     Enable Requestor     Bus B  Bus A    Function       Action     ______________________________________     0      0        No Grants      Requestor                                    Disabled     0      1        Grant Bus A Only                                    Used in a split     1      0        Grant Bus B Only                                    system or in a                                    joined system                                    with only one bus                                    operational     1      1        Grant both buses                                    Used in a joined                                    system     ______________________________________

                  TABLE III     ______________________________________     BUS GRANT ARRANGEMENT IN A JOINED SYSTEM     Bus Busy Condition                    Highest Priority                                2nd Highest     BBSYA  BBSYB       Requester   Priority Req     ______________________________________     Avail  Avail       Grant Bus A Grant Bus B     Avail  Busy        Grant Bus A Grant Bus B     Busy   Avail       Grant Bus B Grant Bus A     Busy   Busy        Grant Bus B Grant Bus A     ______________________________________

Table II indicates the various state values related to each bus request line thus, as seen in Table II, the situation can occur where there are no bus grants, or where there is only one bus which is granted for access or a situation, as in a joined system, where both busses may be enabled for grant and use.

Table III shows the operations for bus grant in a joined system. Here it will be noted that when both busses A and B are available, the highest priority requestor will get a grant to bus A while the second highest priority requestor will get a grant to bus B. Likewise, if system bus A, 22A, is available while system bus B, 22B, is busy, the highest priority requestor will be granted use of bus A. Likewise if only system bus B is available, then the highest priority requestor will be granted bus B.

EQUAL ACCESS ARBITRATION FAIRNESS SCHEME

All the requestor modules on the bus are given equal priority for usage of the system bus. The bus arbiter 18 implements the fairness scheme with a LRU (Least Recently Used) algorithm. The arbiter 18 keeps track of the usage of the system bus by different modules and gives a temporary higher priority to the least recently used module. The priority of the module winning the arbitration is lowered to allow other modules to gain access to the bus. This fairness scheme is implemented as described in a co-pending U.S. patent application entitled "Biased Rotating Priority Arbiter Bus Access System", U.S. Ser. No. 008,943.

(b) Transmit Phase: The transmit phase, while depending on the bus command, consists of either a command/address cycle only or a command/address cycle followed by one or four data cycles. For "message type" commands, the address field in the command/address phase is replaced by a destination ID (Identity) field and the subsequent data words are replaced by message words.

The Transmit Phase follows the Arbitration Phase. If the requestor is granted the bus and the bus is available, the requestor starts driving the bus during this phase. The requestor broadcasts the request information on the bus. The server modules on the bus decode the information to check to see if they are the target or destination for this request. If a module is the target of the bus operation, this module must "acknowledge" the request to indicate its responsibility to receive and process the request.

During, and at the end of, the Transmit Phase, while waiting to receive an acknowledge signal, the requestor also checks for a Not Ready signal (NRDY) and for a Bus Error signal condition from the server module. If it detects a Not Ready condition, then the requestor waits on the bus. If the requestor detects an error condition, it aborts the current transmission and Retries its request.

If the requestor is expecting data to be returned at the end of transmission, then a "Receive Phase" follows the transmit phase. In this case, the requestor, after receiving an acknowledgement for the Transmit Phase, turns its transceivers around and waits for the data return. The server module (memory) can start driving the bus anytime after asserting "acknowledge" for the request. The requestor asserts the "Bus Busy" signal during this entire transaction, thus indicating to other modules that the bus is currently in use.

If the requestor is requesting a one-way transfer on the bus such as a "Message Transfer" or a "Write-To-Memory", (bus commands consisting of Transmit Phase only), the requestor will stop driving the bus after transferring the last data word of the Transmit Phase and then make the bus available for other modules.

(c) Receive Phase: A Receive Phase, depending on the bus command, consists of either one or four data return cycles. After the server module (memory) has acknowledged and processed the request, the server module starts driving the bus. The requestor, in this case, then becomes the receiving module. There is no acknowledgement signal provided from the requestor for the Receive Phase.

DATA COHERENCY

Since the A-11 computer system operates in a multiprocessor environment, with each processor having its own type of cache unit (ST or NST), the shared data blocks in the cache units and in main memory must contain updated information so that the module does not access "stale" data. This requires "data coherency" between the cache units and the main memory.

The system busses 22A, 22B, in addition to providing a path for data access for various modules, also provides for modules to maintain consistent or coherent data. This is accomplished with "spy" logic which monitors the busses. The spy logic 29 (FIG. 2) monitors the system bus for commands that can cause either invalidation or expulsion of data from the cache unit. This type of spy or snooping system has been described U.S. Pat. No. 5,025,365 entitled "Hardware Implemented Cache Coherency Protocol With Duplicated Distributed Directories For High Performance Multi-Processors" and issued on Jun. 18, 1991, assigned to the same assignee as the present disclosure.

The dual system bus 22, supports both the Store-Through (ST) and a Non-Store-Through (NST) environments. Since the operations for data coherency are quite different in the ST and in the NST environments, then, in the dual processor format of the A-11 computer system, the dual system bus 22 will support either one of the two environments but not both at the same time. In the A-11 system, the IMP (28I) based processor (Store-through cache, ST) will follow a Store-Through protocol, while another type of processor designated CHAMP (CMOS Hardware A series Mainframe Processor) (28C) will follow a Non-Store-Through (NST) protocol. The memories 48 and the interbus interface modules 68 (PMIU) are designed to follow a general protocol which is suited for supporting both the Store-Through and Non-Store-Through environments.

In a Store-Through (ST) mode, the Write commands from the requestor modules are always issued to the main memory 48 so that the memory module always has the updated information (master or original data). In order to maintain data coherency, the other modules on the bus containing cache units will monitor the bus for memory Writes and invalidate any copy of that correspondingly particular data that resides in their cache unit. FIG. 3 shows a schematic drawing of the central processing module 8 having various enabling circuitry and the processor 28. Included in the processor logic 6, is a cache unit 28_(i) for Store-Through-Cache and a cache unit 28_(c) for Non-Store-Through Cache. If the processor (Integrated Mainframe Processor) 28 is an IMP processor, 28I, then the cache unit 28_(i) is a Store-Through cache unit. On the other hand, if the processor 28 is a CHAMP processor, 28_(c), then the cache unit 28_(c) is a Non-Store-Through functioning unit.

In a Non-Store-Through (NST) environment, the concept of master/copy is used at a 4-word block level in order to maintain data coherency. In this arrangement, the "Writes" from the processor 28 are always done locally in the cache 28_(a) by acquiring a master (original) block either from the main memory 48 or from another Non-Store-Through (NST) cache unit in the system.

The request for a master block causes the cache unit 28_(a), FIG. 4B, holding the master block, to return the data to the requestor and all other cache units to invalidate any existing copies. The requestor, after acquiring a master block, keeps the block until it is purged out due to lack of space in the cache unit, or else is requested by another module in the system. The Reads from the processor 28 translate into a request for a copy of the block on the system bus. The module holding the master returns the master block to main memory 48 and provides a copy of the block to the requestor module.

There are certain basic rules for "data coherency" in a Non-Store-Through" (NST) environment.

These include:

1. There can be only one master block in the system for each physical address at any time. A master block of data involves a four-word block of data which is original, valid data written into main memory and can only be overwritten by the module that owns this data. In the Non-Store Through (NST) environment, "copies" of this master block can be issued to requesting modules, but these copies cannot be overwritten.

2. After system initialization by Maintenance processor 40_(p) of FIG. 1, the main memory 48 has all master blocks.

3. Copies of a master block are issued to requesting modules as long as the main memory 48 has the master block, with a provision that copies cannot be overwritten.

4. Writes are permitted only after ownership of a master has been obtained with the exception of New Data Writes from the I/0 module 68_(a), 68_(b), (PMIU). When a module requests a master block, copies of the master block in other caches are invalidated. Thus, if the module requesting the master wishes to update the master, then of course, copies of the master in other caches must be invalidated and are done so.

5. A master block can be transferred to a requestor from main memory 48 or from another central processing module 8_(a), 8_(b).

6. The data blocks can move from one cache unit to another cache unit in a Non-Store-Through (NST) environment if the modules are contending for the same master block. In order to prevent the shuttling of blocks between the modules without accomplishing useful work, the requestor of a block first must "inhibit" other requestors from acquiring that same block by asserting the RETRY signal.

Once the master block has been obtained, the requestor continues to assert RETRY until after it has performed the desired operation on that block. This is part of the spying function of the module. After the module has performed the necessary operation, it will either release the block to the requestor on the following RETRY, or purge it back to main memory 48. In the latter case, the contending requestor will eventually get the block from main memory 48.

With there being two system busses in the A-11 computer system, the spying for the request (while inhibiting other requestors from making a request for the same block) will start on the same clock that the requestor is making a request. At the same time that the requestor is transmitting its request for the block, it is also watching the other system bus to see if another requestor is asking for the same block. If it finds another requestor seeking the same block, it asserts "RETRY" on the bus causing the other requestor to RETRY sometime later. In order to prevent a RETRY on both busses, causing both busses to back off, the requestor on system bus A, 22A, is given higher priority.

The module requesting a block using system bus A and spying on system bus B will assert a "RETRY" on the system bus B. The module requesting on system bus B, and spying on system bus A, will not assert the RETRY signal. This will prevent a memory module 48 from processing two requests for the same block latched at the same time on two different busses.

The interbus interface module 68 (PMIU 68_(a), 68_(b)) operates the same way in both the Store-Through (ST) and the Non-Store-Through (NST) environments. For full block updates (with "new data" coming from a peripheral-input operation), the interbus interface module 68 always Writes the block to main memory 48 causing the master block and any copies of the block in cache units to be invalidated. For partial block updates, the interbus module 68 maintains a 4-word block cache unit and acquires the master block in this cache unit. After performing the required function, the interbus module 68 immediately purges the block back to main memory.

The PMIU interbus interface module 68 starts spying on the bus on the same clock that it makes a request for a master data block. After acquiring the master block, and during the time it is holding the master, the interbus module 68 spies on both busses to see if another module is requesting the same block. The interbus interface module 68 (PMIU) stops spying after it has successfully purged the block back to main memory and received an acknowledge signal for it. The interbus interface module (PMIU) 68 has a cache unit 68c, FIG. 1 which will operate differently from the central processor NST cache unit 28c in two functions:

(a) The interbus interface module 68 (PMIU) will not hold the master block after it has performed the desired operation on it, unlike processor NST cache units which continue to hold a master until it is purged or requested by another module;

(b) The interbus interface module 68 (PMIU) will not transfer a master set of data words directly to the requestor. It will always return a master block back to main memory 48. The interbus interface module 68 will "inhibit" requests for the same block until after the block is returned to main memory 48.

SIGNAL DEFINITIONS

The system bus signals can be categorized into four groups:

(a) Arbitration signals (22a, FIG. 2)

(b) Multiplexed command, address and data transfer signal (22_(d), FIG. 2)

(c) Control/Status signals (22_(c), 22_(a), FIG. 2)

(d) Miscellaneous signals (22_(k), FIG. 2)

All signals except bus request lines required for arbitration are duplicated on each bus. The bus request lines are shared by both busses.

(a) ARBITRATION SIGNALS:

(a1) Bus Request Signals (BREQ3₋₋ N, BREQ2₋₋ N, BREQ1₋₋ N, BREQ0₋₋ N)

Source: Processor and PMIU modules

Dest: Bus Arbiter

Polarity: Active Low; can be active for multiple clocks

Type: Unidirectional

Transmit only for processor and PMIU modules

Receive only for module containing Bus Arbiter

A bus request line is dedicated to each module. The arbitration logic 18 monitors the bus request lines of all modules to determine the next bus master. There is only one set of request lines, shared by both system busses. In a joined configuration, the requestor is assigned a bus based on its priority and availability of the busses. In a split system each bus is dedicated to a particular partition group. The system arbiter will limit a requestor's system bus access to the bus assigned to that requestor's partition. The bus requestor keeps the "Bus Request" signal active until it detects Bus Grant (BGNTn) and bus free conditions, then drives the bus. The Bus Request is deactivated on the same clock the requestor gets to drive a command.

(a2) BUS GRANT SIGNALS (BGNT3₋₋ N, BGNT2₋₋ N, BGNT1₋₋ N, BGNT0₋₋ N)

Source: Bus Arbiter

Dest: Processor and PMIU modules

Polarity: Active Low; can be active for multiple clocks

Type: Unidirectional

Receive only for processor and PMIU modules

Transmit only for module containing Bus Arbiter

Each module on the bus is provided with two dedicated bus grant lines (one for each bus 22A and 22B) from the arbitration logic 18. It indicates to the requestor that it has ownership of the bus on the following clock if the bus is "not busy". The requestor may get the bus grant in the same clock in which it makes a bus request, if no higher priority requestors are requesting the bus. Also the requestors who are granted the bus but unable to transmit requests on the bus due to a bus busy condition may lose their grant before the bus becomes free if any higher priority requestors are requesting the bus.

Since there are two busses in the system in a dual system configuration, the arbitration logic grants the selected requestor the first bus that becomes available. In a split or partitioned system, the bus arbiter only grants, to a requestor, the bus assigned to that requestor's partition. The bus arbiter keeps the bus grant signal active as long as the bus request (BREQn) signal from the requestor is active.

(a3) Bus Busy (BBSY₋₋ N)

Source: Bus Master module

Dest: Other Processor and PMIU modules

Polarity: Active Low; can be active for multiple clocks

Type: Bidirectional

The current bus master (module that has been granted bus access) asserts this signal to indicate a "Bus Busy" condition to other modules on the bus. The module asserts this signal on the clock following the detection of the bus grant and "Bus-Free" (not bus busy)conditions. The bus master module keeps the signal (Bus Busy) active during the entire transaction. If the bus transaction includes a request and a return phase, the module expecting a data return following a request phase will keep the bus busy condition active until it receives the requested information or detects bus error or retry bus conditions.

Each requestor module monitoring "Bus Busy" condition and seeking to get on the bus, must be capable of timing out on this signal. If the signal is not deactivated after some large period of time specified by a time-out value, the module will report error to a Maintenance Processor 40_(p) (FIG. 1). The time-out value for the module will be set during the module initialization.

(b) COMMAND/ADDRESS AND DATA TRANSFER SIGNALS

(b1) ADR₋₋ DATA (63:0) Signals

Source: Processor, PMIU (Processor Message Interface Unit) and memory modules

Dest: Processor, PMIU and memory modules

Polarity: Active High; command/address asserted for 1 clock only; data can be valid for multiple clock cycles

Type: Bidirectional

These bidirectional signals are used to transfer information such as commands, address and data between the modules on the bus. The signals multiplex the command/address phase with the data phase. A separate status signal on the bus "CMDVAL" differentiates the command/address phase from the data phases. In the "command" phase, it transfers either 32 bits of memory address or 32 bits of destination ID field and a command byte. In the "data" phase, it transfers either 6 bytes of E-Mode memory data or 6 bytes of message word and a 4-bit E-mode tag associated data or message word. Each byte of information on these lines is covered by a separate parity signal. The signal meanings for each phase are listed below in Table III.

                  TABLE III     ______________________________________     BITS   COMMAND PHASE       DATA PHASE     ______________________________________     63:56  unused              unused     55:52  unused              unused     51:48  Don't Care with valid parity                                E-Mode data tag     47:40  Don't Care with valid parity                                data/msg byte 5                                (MSB)     39:32  command byte        data/msg byte 4     31:24  address/destin ID byte 3(MSB)                                data/msg byte 3     23:16  address/destin ID byte 2                                data/msg byte 2     15:08  address/destin ID byte 1                                data/msg byte 1     07:00  address/destin ID byte 0(LSB)                                data/msg byte 0                                (LSB)     SIGNAL MEANINGS FOR ADR.sub.-- DATA (63:0)     DURING COMMAND AND DATA PHASES     ______________________________________

(S1) BIDIRECTIONAL LINES DURING COMMAND/ADDRESS (DESTINATION ID) PHASE:

31:0! Address (Destination ID) bits 0 to 31:

For memory type commands, these lines provide the 32 bits (4 bytes) of physical E-mode address. For message transfer commands (Send Message and Read TOD), these lines provide 32 bits of destination ID field. Bits 7:00! are the least significant byte and bits 31:24! are the most significant byte of the address (or destination ID) field. The destination module ID is extracted from word 0 of the message block formed by the MCP (Master Control Program) during the External Function (XFUNC) operator. The E-Mode address refers to specialized E-Mode Systems which use microcode emulation of a set of operators. Thus much hardware formerly used to perform operators is replaced by microcode. Table IV shows four destination modules which can be coded to identify the destination module.

                  TABLE IV     ______________________________________     DESTINATION ID FIELD      03:00!           DESTINATION MODULE     ______________________________________     Choice Open       Processor 0     Choice Open       Processor 1     Choice Open       PMIU 0     Choice Open       PMIU 1     DESTINATION ID FOR BUS REQUESTOR MODULES     ______________________________________

39:32! Command field:

These lines provide the encoding for the memory or message type command to be executed. Table V shows the Command Set for a Store-Through (ST) environment while Table VI shows the Command Set for a Non-Store Through (NST) environment.

                  TABLE V     ______________________________________     (STORE-THROUGH)     COMMAND         SOURCE       DEST     ______________________________________     Read Word       IMP          Memory     Shared Read     PMIU         Memory     Private Read    PMIU         Memory     Write Word      IMP          Memory     Purge Block     PMIU         Memory     New Data Write  PMIU         Memory     Write SP Word   IMP          Memory     Write Protect Word                     IMP          Memory     Read Lock       IMP          Memory     Read TOD        IMP          PMIU     Send Message    IMP, PMIU    IMP, PMIU     COMMAND SET - STORE THROUGH ENVIRONMENT     ______________________________________

                  TABLE VI     ______________________________________     (NON-STORE-THROUGH)     COMMAND      SOURCE        DEST     ______________________________________     Shared Read  CHAMP, PMIU   Memory, Cache     Private Read CHAMP, PMIU   Memory, Cache     Purge Block  CHAMP, PMIU   Memory, Cache     New Data Write                  PMIU          Memory     Read TOD     CHAMP         PMIU     Send Message CHAMP, PMIU   CHAMP, PMIU     COMMAND SET - NON-STORE-THROUGH     ENVIRONMENT     ______________________________________

Table VII is an illustration of the Command Encoding for the eleven Commands listed.

                  TABLE VII     ______________________________________               ENCODING                                          34                 39                       WD/   33   32*     COMMAND     C/M    38    37  36  35  BL    WR   RD     ______________________________________     Read Word   0      0     0   0   0   0     0    1     (RDWD)     Shared Word(SR) 0                 0      0     0   0   1   0     1     Private Read(PR)0                 0      0     0   1   1   0     1     Write Word  0      0     0   0   0   0     1    0     (WRWD)     Purge Block     (PURGE)     0      0     0   0   0   1     1    0     New Data Over     Write (NDOW)                 0      0     0   0   1   1     1    0     Write SP Word     (WRSPWD)    0      0     0   0   0   0     1    1     Write Prot Word     (WRPWD)     0      0     0   0   1   0     1    1     Read        0      0     1   0   0   1     1     Lock(RDLK)0     Read        0      0     0   0   0   0     1     TOD(RTOD)1     Send Message     (SNDMSG)    1      0     0   0   0   0     1    0     COMMAND ENCODING     ______________________________________      *NOTES: Table VII      Bit 32 -- 1 Read only type command      Bit 33 -- 1 Write only type command      Bit 34 = 0 Word level command      Bit 34 = 1 Block level command      Bit 39 = 0 Memory type commands      Bit 39 = 1 Message type commands

(S2) BIDIRECTIONAL LINES DURING DATA (MESSAGE) PHASE:

47:0! Data (Message) word bits 0 to 47:

These lines (multiplexed with command/address lines) carry the 48 bits (6 bytes) of the E-mode data word for memory commands and 48 bits of message word for message type commands. Bits 7.0! mark the least significant byte and bits 47:40! mark the most significant byte of the data (message) word field.

51:48! Data (Message) Tag Field:

These lines carry the E-mode tag for data or message words transferred on lines 47:0!. For message words, the tag value is always zero.

63:52! Unused

(b2) PAR(7:0) SIGNALS (PARITY)

Source: Module driving the bus (Processor, PMIU, Memory)

Dest: Receiver modules (Processor, PMIU, Memory)

Polarity: Active High; command/address parity asserted for 1 clock only; data (message) parity must be valid while data is valid on the bus; covers ODD parity

Type: Bidirectional

These bidirectional signals carry odd parity on a byte level (the total number of 1's in the byte including parity must be odd) for signals on ADR₋₋ DATA(51:0) lines. During the command address phase, the signals cover parity for 32 bits of address or destination ID and command fields. During the data or message phase, it covers parity on 48 bits of data or message and 4 bits of tag. The signal meanings for each phase are listed below in Table VIII.

                  TABLE VIII     ______________________________________     SIGNAL    COMMAND PHASE   DATA PHASE     ______________________________________     PAR(7)    Unused          Unused     PAR(6)    Valid Parity    parity for data or                               message tag     nibble     PAR(5)    Valid Parity    parity for data/msg                               byte 5     PAR(4)    Parity for command                               parity for data/msg               byte            byte 4     PAR(3)    Parity for address or                               parity for data/msg               destination ID byte 3                               byte 3     PAR(2)    parity for address or                               parity for data/msg               destination ID byte 2                               byte 2     PAR(1)    parity for address or                               parity for data/msg               destination ID byte 1                               byte 1     PAR(0)    parity for address or                               parity for data/msg               destination ID byte 0                               byte 0     SIGNAL MEANING ON PAR (7:0)     DURING COMMAND AND DATA PHASES     ______________________________________

(c) CONTROL/STATUS SIGNALS

(c1) Command Valid (CMDVAL₋₋ N)

Source: Requestor modules (Processor, PMIU)

Dest: Target modules (Processor, PMIU, Memory)

Polarity: Active Low; asserted for 1 clock only

Type: Bidirectional for Processor and PMIU modules

Unidirectional for Memory modules (Receive only)

Since the ADR₋₋ DATA(63:0) multiplexes the command/address with data cycles, this signal is used to differentiate between these two cycles. The requestor asserts this signal to indicate to receivers that a valid command (memory or message type) and address (or destination ID) information is on the bus. The receiving modules, after detecting this signal, latch the command and address information of the bus and check to see if they are selected as a target. The modules selected as targets for the request continue to monitor the bus and receive further transmission if any. Other modules ignore the bus until they see CMDVAL₋₋ N again.

(c2) Not Ready (MM₋₋ NRDY₋₋ N,PE₋₋ NRDY₋₋ N)

Source: MM₋₋ NRDY₋₋ N:--memory modules; PE₋₋ NRDY N:--processor and PMIU modules. The acronym MM refers to Main Memory, while the acronym PE refers to Processor Element.

Dest: Requestor and other targets (Processor, PMIU, Memory)

Polarity: Active Low; can be active for multiple clock cycles

Type: Bidirectional; Open Collector

The server module (memory) on the bus asserts this signal to indicate to the requestor module that it is not ready for communication on the bus. It can be asserted only after the communication link between the modules on the bus is established.

When a requestor makes a request, the receiving module or modules (including spy modules) on the bus can assert this signal only after having latched in the command and decoded the information to find that it has been selected as a target for the request. The signal is asserted any time after latching the command and before asserting acknowledge (MM₋₋ ACK₋₋ N or PE₋₋ ACK₋₋ N) or the receipt of the request. The requestor, after transmission, monitors the NRDY₋₋ N signal and delays further transmission accordingly. For the bus transactions containing only a command/address cycle in the transmit phase, this signal can be asserted to delay both acknowledgement or data/message transfers. The signal causes the requestor to either hold the current information (data or message word) on the bus if it has already begun transmission or delay it until the module goes ready again.

In the case when the request has multiple targets (e.g. a Private Read request has both memory and processors as targets), a target that is not asserting NRDY₋₋ N must monitor the signal to see if another target has asserted the signal and must delay its acknowledgement signal accordingly.

The bus interface logic on each module latches the NRDY₋₋ N signal before deciding to transmit. Therefore, the transmitter will hold or change information on the bus on the clock following the detection of a Not Ready or a Ready condition.

The memory module 48 uses this signal to delay the acknowledgement of an MM₋₋ ACK₋₋ N signal and to control the flow of write-data words coming from the requestors. The processor modules will use this signal to delay the acknowledgement signal PE₋₋ ACK, if more time is needed to determine if the requested information is in cache; and, to control the flow of invalidation requests, (a deadlock condition can occur if two IMP based processors are writing to memory, both have their invalidation queue full, and both go "not ready" on each other) and to control the flow of message words from PMIU modules 68_(a), 68_(b). The PMIU modules will use this signal to control the flow of message words returned from the processor or other PMIU modules.

The NRDY₋₋ N signal can not be used in the receive phase of the bus transaction to delay data return words coming from either memory or cache. Once the requestor has made request for the data, it must take data words at memory or cache speed.

Each module on the bus monitoring the NRDY₋₋ N signal (these modules are target modules that did not assert NRDY and did not acknowledge the requestor module) must be capable of Timing Out on the signal. If the signal is not deactivated after some large period of time specified by a (settable time-out value, the module reports error to the Maintenance Processor 40_(p). The time-out value is set during module initialization.

This signal (NRDY₋₋ N) cannot be used to delay or hold the command/address cycle of the transmission. It mainly provides the control for the server module (memory) to delay the request processing (once it is accepted) and delay transmission of data words from the requestor. Also with two system busses in the network, there can be a deadlock condition when two requestors send requests at the same time on two different busses and go "Not Ready" on each other. The requestor on System Bus A is given higher priority. The requestor on bus B will back off and retry its request later. This scheme will cause only one requestor (on System Bus B) to back off.

(c3) Data Valid (DATAVAL₋₋ N)

Source: Transmitter module of data or message words (Processor, PMIU, Memory)

Dest: Receiver module of data or message words (Processor, PMIU, Memory)

Polarity: Active Low; asserted while information is valid on the bus

Type: Bidirectional

The modules on the bus assert this signal to indicate to the receiver that the current information on the bus ADR₋₋ DATA(63:0) and PAR(7:0) is valid. The receiver uses the DATAVAL₋₋ N signal to latch data or message words on the bus. The signal can be asserted only during data or message word transfers on the transmit and receive phases. It also allows the Transmitter module to control the pace of transmission when sending data or message words. If the Receiver goes "Not Ready" while the transmitter is driving valid data on the bus, the transmitter will hold information on the bus (with DATAVAL₋₋ N asserted) for consecutive cycles until the receiver goes Ready again.

(c4) Memory Acknowledge (MM₋₋ ACK₋₋ N)

Source: Memory modules

Dest: Memory Requestor (Processor, PMIU)

Polarity: Active Low; asserted for 1 clock only

Type: Unidirectional; common to all memory modules

Transmit only for memory modules

Receive only for Processor and PMIU modules

The memory modules 48 assert this common memory "Acknowledge" signal to acknowledge the receipt of the transmission, and indicate to the requestor, possession of the requested information and responsibility for processing the request. The memory module can assert this signal only after decoding the command and address mapping information and determining that it is the target of the request. The signal is asserted in the 2nd clock (if not delayed by NRDY) following the end of a transmit phase. The transmit phase may include only the command/address cycle for memory read requests, a command/address cycle followed by a data cycle for 1 word Memory Write requests or a command/address cycle followed by four data cycles for 4 word Memory Write requests.

If the memory module is busy processing other requests and cannot assert MM₋₋ ACK₋₋ N in the 2nd clock following the transmit phase, it can go Not Ready in the clock following the end of the transmit phase and delay MM₋₋ ACK₋₋ N. In this case, the MM₋₋ ACK-N signal is asserted in the clock after the memory module becomes Ready again. Also in the case where the memory module is ready to process the request but some other target (such as a processor's cache invalidation logic or NST cache) is not ready to process, the memory module will monitor the NRDY₋₋ N signal and delay its MM₋₋ ACK₋₋ N accordingly until it recognizes the Ready signal again.

If a memory module receives valid command and address fields but the address does not map on to the memory module, it will stop receiving information from the bus and will not assert the acknowledgement signal.

If the requestor drives an address that does not map into any of the memory modules of the NST caches of the CHAMP 28c processor modules on the bus, the requestor will not see either MM₋₋ ACK₋₋ N or PE₋₋ ACK₋₋ N and interprets the lack of any acknowledgement signals as an invalid address.

The memory module can drive requested information on the bus any time after asserting MM₋₋ ACK₋₋ N. The requestor, after detecting MM₋₋ ACK₋₋ N, turns its transceivers around to receive information on the bus.

Acknowledgement is given for the whole transmit phase or transmission packet and not for individual cycles in the transmit phase. Also there is no acknowledgement for the receive phase, that is, when memory is returning requested information, the requestor does not acknowledge.

(c5) Processing Element Acknowledge (PE₋₋ ACK₋₋ N)

Source: Processor (CHAMP) containing NST cache, PMIU modules

Dest: Memory/NST Cache Requestor (Processor, PMIU)

Polarity: Active Low; asserted for 1 clock only

Type: Bidirectional for Processor and PMIU modules

Unidirectional (Receive only) for memory modules

The processor (CHAMP) containing a private NST cache, and the PMIU modules (68a, 68b) assert this common processing element "Acknowledge" signal to acknowledge the receipt of the transmission and indicate to the requestor that it has the requested information and is responsible for processing the request.

The processor module asserts this signal for both memory commands (Private Read, Shared Read) and message type (Send Message) commands while the PMIU module asserts it only for message type (Read TOD, Send Message) commands. For memory type commands, the processor asserts "Acknowledge" after decoding the request and checking the cache tag directory to see if the requested information is in the local cache. For message type commands, the modules assert "Acknowledge" after decoding the message command and destination ID to see if they are the target for the request. The signal is asserted in the 2nd clock (if not delayed by NRDY) following the end of a transmit phase.

If the target module is busy or needs more time to check the cache tag directory or process a message request, and cannot assert PE₋₋ ACK₋₋ N in the 2nd clock following the transmit phase, it can go Not Ready in the clock following the end of transmit phase and delay the acknowledge response. In this case, the PE₋₋ ACK₋₋ N signal is asserted in the clock after the processor module goes Ready again. Also in the case where there are multiple targets for the request and one target is ready but the other target is not ready to process the operation, the target that is ready to process will monitor the NRDY₋₋ N signal and delay its PE₋₋ ACK₋₋ N accordingly until it sees Ready again. For example, a Private Read request has both processor and memory as targets and if the processor is ready but memory is not ready to process the operation, the processor will monitor RDY₋₋ N and delay PE₋₋ ACK₋₋ N accordingly.

For memory type commands, if the processor receives valid command and address fields but the address is not located in the cache tag directory, it will not assert acknowledge. For message type commands, if the destination ID does not match the module's ID, the module will not assert acknowledge.

If the requestor drives an address that does not map in any of the NST caches on any processor module or in any memory module, the requestor will not see either PE₋₋ ACK or MM₋₋ ACK and interprets the lack of any acknowledge signal as an invalid address.

The target module can drive requested information on the bus any time after asserting PE₋₋ ACK₋₋ N. The requestor, after detecting PE₋₋ ACK₋₋ N, turns its transceivers around to receive information on the bus.

The acknowledge is asserted once for the whole transmit phase or transmission packet and not for individual cycles in the transmit phase. Also there is no acknowledgement for the receive phase, that is, when the processor or PMIU is returning requested information, the requestor does not acknowledge.

PE₋₋ ACK₋₋ N causes a memory module to ignore or abort the processing of the request associated with the same bus cycle, since the processing element concurrently targeted as receiver will now fulfill the request.

(c6) Retry (RETRY₋₋ N)

Source: Receiver of the request transmission (Processor, Memory, PMIU)

Dest: Requestor (Processor, PMIU) and other targets that monitor bus

Polarity: Active Low; asserted for 1 clock only

Type: Bidirectional (Open Collector)

The signal is asserted by the receiver of the request to indicate to the requestor to abort the current transmission and retry its request. The retry is asserted by a module in the following cases:

1. Detection of a parity error on the address or destination ID field; multiple modules can assert this error since all of them receive the command/address cycle;

2. Detection of a parity error on the command field;

3. Detection of a parity error on the data field and the address or destination ID field has confirmed the module to be a target;

4. Detection of other module related internal errors when receiving transmission and the address or destination ID field has confirmed the module to be a target;

5. If the module is currently busy (queue full condition or spy logic) processing other requests and the address or destination ID field confirms the module as a target;

The signal is asserted by the module in the 2nd clock following a command/address cycle, data or message transfer cycles. The signal causes a requestor to wait for a certain amount of time (specified by the retry waiting period), contend for the bus again and retry its request.

The retry signal is asserted both for handling error situations and resolving data coherency. However, with the error conditions, the Bus Error signal is also asserted. This distinguishes the two cases of retry situations.

In a case where the request has multiple targets (e.g. Private Read request has memory and processor as targets), the target that is not asserting RETRY₋₋ N must monitor the signal to see if another target has asserted the signal. All targets involved in a RETRY₋₋ Nbus cycle will ignore the entire transmission.

Each requestor on the bus maintains two states related to the retry features. These are Retry Waiting Period and Retry Limit. The Retry Waiting Period specifies the amount of time the requestor must wait before retrying its request. The Retry Limit keeps track of the number of retries performed by the requestor for a particular request and the maximum number of retries allowed before reporting an error to the originator of the request or the Maintenance Processor 40_(p). The Retry Waiting Period and Retry Limit values are programmable and can be set to a fixed value during the initialization process. The Retry Limit is reset to its initial value every time a new request is made on the bus and the retry limit is decremented for each retry of that request.

The value of the Retry Waiting Period to be set can be adjusted to various situations. If the retry feature is used to solve some of the deadlock situations on the bus, then different modules can be set with different Retry Waiting Period values so that they do not try their requests again at the same time. Another solution is to use random number generator logic for the Retry Waiting Period and to initialize each module with different seed values.

The Retry Limit can be set to some high number within any MCP (Master Control Program) time-out limit, allowing a requestor to perform a sufficient number of retries before reporting an error to the Maintenance Processor, 40_(p).

In an NST PE (Non-Store-Through Processing Element) environment where the master data block is in the PE transmit queue, asserting retry could result in a "deadlock" condition if the requested master block is forced to wait on the successful transmission of its previous request (which could be receiving retry from another PE because the master it is requesting, is in the queue). Therefore, the hardware on an NST PE must be implemented to either support direct access of the master from queue or separate queues for purges and requests, or out of order transmission of requests.

The retry feature is not supported for a "receive phase" (data return phase) of bus transaction for the following reasons:

1. To allow overlapping of a current bus transaction (last word of data transfer) with the arbitration of the next bus master and the start of the next bus transaction on the following clock.

2. In a ST environment, where a master data block is always read from the main memory (assuming that the memory does not keep a `valid`, `invalid` state for the block). If the requestor detects an error on data returned from memory, it can retry the whole transaction and read the master again.

3. However, in an NST environment, this scheme may not work as the master may be transferred from the NST cache of another processor. If the retry feature is supported in the receive phase and if the requestor detects an error on the first three words of data transfer, it can assert retry and cause cache to transfer the master block again. However, if the system bus interface logic forwards data to the CPU/Cache in parallel with receiving information on the bus, then since part of the block is already transferred to the CPU/Cache, the rest of the block needs to be transferred with the "data corrupt" signal. When the requestor retries the request and gets the master block again, it needs to overwrite the previous master block sent to CPU/Cache. The CPU/Cache needs to support this feature. If an error is detected on the fourth data word, the next bus transaction has already begun. The NST cache that returned the block must also change its internal state for the block as `Not Present` and at this point it is too late to assert retry to cause the cache to transmit again. If the requestor tries to retry the whole transaction (since the cache does not have the block) a stale data block will be returned by the memory module.

The simple option for the requestor in an NST environment is to receive data words without causing memory or other caches to retry the transmission and set the data corrupt signal for the ultimate destination--CPU/Cache or PMIU logic. At some later point, when the CPU or PMIU logic accesses the corrupted word, it will abort the operation.

(c7) Bus Error (BERR₋₋ N)

Source: Target and spying modules (Processor, Memory, PMIU)

Dest: Requestor (Processor, PMIU) and other targets that monitor bus

Polarity: Active Low; asserted for 1 clock only

type: Bidirectional (Open Collector)

The signal is asserted by the receiver modules, including spy modules to indicate to the requestor of an error in the previous bus transmission. It causes the requestor to abort transmission of its request. The signal is asserted by a module if it detects:

1. A parity error on address or destination ID field; multiple modules can assert this error since all of them receive the command/address cycle;

2. A parity error on the command field;

3. A parity error on the data field and the address or destination ID field has confirmed the module as a target;

4. Other module related internal errors when receiving a transmission and the address or destination ID field has confirmed the module as a target.

The Bus Error signal is asserted by the module in the 2nd clock following the command/address cycle, the data cycle or the message transfer cycle. If the retry signal is asserted along with Bus Error, it will cause the requestor to wait for a certain amount of time (specified by the retry waiting period), contend for the bus and retry its request.

(c8) Corrupted Data (CORRPT₋₋ DATA₋₋ N)

Source: Modules driving data or message words (Processor, Memory, PMIU)

Dest: Receiver modules (Processor, Memory, PMIU)

Polarity: Active Low; asserted while data or message word is valid on the bus

Type: Bidirectional

This signal has meaning only in the data or message transfer phases and is ignored by the modules during the command/address phase. This signal, when asserted during the data or message phase, indicates to the receiver that the data or message word on the bus is corrupted. The signal is asserted along with DATAVAL₋₋ N and information on the bus. The module or unit that detects an error in a data word sets this bit and transfers the word onto its destination. It also reports this error to the maintenance processor. The receiver of this data continues operation as normal and does not report this error to the maintenance processor.

(c9) Corrupt Data Parity (CPARITY)

Source: Modules driving CORRUPT₋₋ DATA₋₋ N signal (Processor, Memory, PMIU)

Dest: Receiver modules (Processor, Memory, PMIU)

Polarity: Inverted Corrupt Data signal; valid while data is valid on the bus

Type: Bidirectional

This signal caries inverted CORRUPT₋₋ DATA₋₋ N signal and has meaning only in data or message transfer phases.

(d) Miscellaneous signals:

(d) SYSTEM CLOCK (CLOCK)

Source: Maintenance processor

Dest: All modules on the bus (Processor, PMIU, Memory)

Polarity: 16 Mhz

Type: Unidirectional

The modules on the system bus synchronize their operations to the system clock.

(d2) SYSTEM RESET (RESET)

Source: Maintenance processor

Dest: All modules on the bus (Processor, PMIU, Memory)

Polarity: Active Low

Type: Unidirectional

When the signal is active, all modules on the bus reset their system bus interface logic. The current bus activity and pending requests at the bus interface are aborted.

(d3) MICRO SECOND CLOCK (CLK₋₋ 2.4 s)

Source: Maintenance processor

Dest: PMIU modules

Polarity: 2.4 μs (micro seconds)

Type: Unidirectional

The PMIU modules uses this clock to update and maintain a TOD (Time of Day) counter. The TOD information returned to the requestor (MCP) is in the units of 2.4 microseconds.

(d4) 512 MICRO SECOND CLOCK (CLK₋₋ 512 s)

Source: Maintenance processor

Dest: Processor modules

Polarity: 512 μs (micro seconds)

Type: Unidirectional

COMMAND DESCRIPTION: The following commands are listed and described operationally for enablement on the system busses 22A, 22B:

READ WORD (RDWD)

Source: IMP based processor modules

Dest: Memory module

Type: ST, memory type, request followed by 1 word data return

Cycles: Transmit Phase: 1

Receive Phase: 1

Processor: The IMP based processor module (28I_(a), 28I_(b), FIG. 4A) issues this command on the bus to fetch 1 word of data (or code) from the memory module. After transmitting the request, the requestor waits on the bus for either an acknowledge followed by the requested information (DATACVAL₋₋ N) or for Not Ready or Bus Error or Retry conditions. If the processor has requested a code word, it will check for a valid code tag (tag=3) after receiving the word.

Memory: After receiving the request, one of the memory modules containing the requested information acknowledges the request. When memory has finished processing the request, it asserts dataval and transfers the information to the requestor. If the memory module detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with dataval and corrupted data.

PMIU: The PMIU module does not issue this command, but spies on the bus for this command. If it is holding the requested information as a master, it will assert retry causing the memory module to abort processing and causing the processor module to retry its request to the PMIU module.

WRITE WORD (WRWD)

Source: IMP based processor modules

Dest: Memory module

Type: ST, memory type, request only (one way transfer)

Cycles: Transmit Phase: 2

Receive Phase: 0

Processor: The IMP based processor module issues this command on the bus to write 1 word of data into the memory module. The 1 word of data to be written into memory follows the command/address cycle. After transmitting the request, the requestor waits on the bus for either acknowledgement or Not Ready or Bus Error or Retry conditions. The other processor module's spy logic monitors the system bus for this command and issues invalidations to its ST cache.

Memory: After receiving the request, one of the memory modules containing the requested address acknowledges the request on the bus. It causes a memory module to unconditionally write (overwrite) the data word at the requested address. If the memory module detects a data parity error on the data word sent by the IMP or runs into any internal processing errors while processing the request (after it has asserted acknowledge on the bus), it writes data into the memory with the data corrupt bit on.

PMIU: The PMIU module does not issue this command, but spies on the bus for this command. If the PMIU is holding the master of the requested information, it will assert retry causing the memory module to abort processing and causing the processor module to retry its request to the PMIU.

WRITE PROTECT WORD (WRPWD); WRITE SINGLE PRECISION WORD (WRSPWD)

Source: IMP based processor modules

Dest: Memory module

Type: ST, memory type, request followed by 1 word data return

Cycles: Transmit Phase: 2

Receive Phase: 1

Processor: The IMP based module issues this command on the bus to write 1 word of data to the memory module conditionally and return the initial contents of the addressed location (optional if no tag error). The 1 word of data to be written in memory follows after the command/address cycle. After transmitting the request, the requestor waits on the bus for either an acknowledgement followed by the requested information along with ETAG₋₋ ER or Not Ready or Bus Error or Retry conditions.

The other processor module's spy logic monitors the system bus for this command and issues invalidations to its ST cache.

Memory: After receiving the request, one of the memory modules containing the requested address acknowledges the request on the bus. It causes a memory module to read the data at the requested address and to write new information from the processor only if the data tag for the word read satisfies the E-Mode protection requirement. When memory has finished processing the request, it asserts dataval and transfers the original contents of the addressed location to the requestor. If the data tag for the word read does not meet E-Mode protection criteria, the memory module also asserts ETAG₋₋ ERR signal along with dataval and requested data word. The transfer of the original contents of memory is optional if the tag is ok as long as dataval and unknown data with good parity are driven on the bus. If a memory module detects any uncorrectable errors (other than the E mode tag match error) while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with dataval and corrupted data.

PMIU: The PMIU module does not issue this command, but spies on the bus for this command. If it is holding the master of the requested information, it will assert retry causing the memory module to abort processing and the processor module to retry its request.

READ LOCK (RDLK)

Source: IMP based processor modules

Dest: Memory module

Type: ST, memory type, request followed by 1 word data return

Cycles: Transmit Phase: 2

Receive Phase: 1

Processor: The IMP based processor module issues this command on the bus to perform an indivisible operation to store a value into a word and return original contents. The processor supplies the data word to be written in memory and requests the original contents of the addressed location. The 1 word of data to be written in memory is transmitted following the command/address cycle. After transmitting the request, the requestor waits on the bus for either an acknowledgement followed by the requested information or a Not Ready or a Bus Error or a Retry conditions. The other processor module's spy logic monitors the system bus for this command and issues invalidations to its ST cache.

Memory: After receiving the request, one of the memory modules containing the requested address acknowledges the request on the bus. It causes the memory module to read the data at the requested address and to write new information from the processor only if the data tag for the word read satisfies the E-Mode protection requirement. The memory module must perform the operation: read followed by write, indivisibly. When memory has finished processing the request, it asserts dataval and transfers the initial contents of the addressed location to the requestor. If the data tag for the word read does not meet E-Mode protection criteria, the memory module also asserts ETAG₋₋ ERR signal along with dataval and requested data word and aborts the store part of the cycle. If the memory module detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with the dataval signal and the corrupted data.

PRIVATE READ (PR)

Source: CHAMP processor (with NST cache) and PMIU modules

Dest: Memory or CHAMP processor (with NST cache) modules

Type: Used by PMIU in both ST and NST environments, used by processor modules in NST environment only, memory type, request followed by 4 word data return

Cycles: Transmit Phase: 1

Receive Phase: 4

Processor: The CHAMP based processor (28C_(a), 28C_(b), FIG. 4B) with its NST cache issues this command on the bus to fetch a 4-word block of data as a master from either the memory module or another processor module's NST cache. After transmitting the request, the requestor waits on the bus for either acknowledge (MM₋₋ ACK or PE₋₋ ACK) followed by the requested information (dataval) or a Not Ready or a Bus Error or a Retry condition.

The processor module's spy logic monitors the system bus for this command either from the PMIU or the other processor module. If the spying processor module has a ST cache, the spy logic issues invalidation to its cache. If it has NST cache and the requested block is resident in the cache, the spy logic issues a request for a master block to the cache. When the NST cache has finished processing the request, it transfers the 4-word block one word at a time with the requested word in the block first. The cache asserts dataval for each word transferred to the requestor. If the cache detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with dataval for corrupted data word in the clock.

Memory: After receiving the request, one of the memory modules containing the requested information acknowledges the request. If the memory module detects PE₋₋ ACK from the processor module indicating that the NST cache has the block, it aborts processing of the request. When memory has finished processing the request, it transfers the 4-word block one word at a time with the requested word in the block first. The memory asserts dataval for each word transferred to the requestor. If the memory module detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with dataval for the corrupted data word in the block.

PMIU: The PMIU module issues this command on the bus to fetch a 4-word block of data as a master from either the memory module or a processor module's NST cache. After transmitting the request, the requestor waits on the bus for either acknowledge (MM₋₋ ACK or PE₋₋ ACK) followed by requested information (dataval) or Not ready or Bus Error or Retry conditions.

The PMIU module's spy logic monitors the system bus for this command either from the processor or other PMIU modules. If the spying PMIU is holding the block as a master, it will assert retry causing a memory module to abort processing and a processor module to retry its request.

SHARED READ (SR)

Source: CHAMP processor (with NST cache) and PMIU modules

Dest: Memory or CHAMP processor (with NST cache) modules

Type: Used by PMIU in both ST and NST environments, used by processor modules in NST environments only, memory type, request followed by 4-word data return

Cycles: Transmit Phase: 1

Receive Phase: 4

Processor: The CHAMP based processor with a NST cache issues this command on the bus to fetch a 4-word block of data (or code) as a copy from either the memory module or another processor module's NST cache. After transmitting the request, the requestor waits on the bus for either acknowledgement: MM₋₋ ACK or PE₋₋ ACK, followed by the requested information (dataval) or Not ready or Bus Error or Retry conditions. The processor will check for a valid code tag (tag=3) since it uses a word out of the code block.

The processor module's spy logic monitors the system bus for this command either from the PMIU or another processor module. If the spying processor module has an ST cache, the spy logic ignores the command. If it has NST cache and the requested block is resident in the cache as master, the spy logic issues a request for a copy of a block to the cache. When the NST cache has finished processing the request, it transfers the 4-word block, one word at a time with the requested word in the block first. The cache asserts dataval for each word transferred to the requestor. If the cache detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus) it asserts the data corrupt signal along with dataval for corrupted data word in the block. After the cache has transferred a copy block to the requestor, it purges the master block back to the main memory. The copy and master blocks are transferred in two separate transactions on the system bus.

Memory: After receiving the request, one of the memory modules containing the requested information acknowledges the request. If the memory module detects PE₋₋ ACK from the processor module indicating the NST cache has the master, it aborts processing of the request. When memory has finished processing the request, it transfers the 4-word block, one word at a time with the requested word in the block first. The memory asserts dataval for each word transferred to the requestor. If a memory module detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with dataval for the corrupted data word in the block.

PMIU: The PMIU module issues this command on the bus to fetch a 4-word block of data as a copy block from either the memory module or processor module's NST cache. After transmitting the request, the requestor waits on the bus for either acknowledgement (MM₋₋ ACK or PE₋₋ ACK) followed by the requested information (dataval) or Not ready or Bus Error or Retry conditions.

The PMIU module's spy logic monitors the system bus for this command either from the processor or other PMIU modules. If the spying PMIU is holding the block as a master, it will assert retry causing the memory module to abort processing and causing a processor module to retry its request to the PMIU holding the block.

NEW DATA OVER WRITE (NDOW)

Source: PMIU modules

Dest: Memory modules

Type: Used by PMIU in both ST and NST environments, memory type, request only (one way transfer)

Cycles: Transmit Phase: 5

Receive Phase: 0

Processor: The processor module does not issue this command, but spies on the bus for this command. It causes the processor module's spy logic to invalidate any master or copy blocks present in its ST or NST cache.

Memory: After receiving the request, one of the memory modules containing the requested address acknowledges the request on the bus. The command causes the memory module to unconditionally write (overwrite) the master block at the requested address. If a memory module detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it writes data into the memory with data corrupt bit on, for the corrupted data word in the block.

PMIU: The PMIU module issues this command on the bus to write a new master block (read from peripheral device) in to the main memory. The 4-word block to be written in memory follows a command/address cycle. After transmitting the request, the requestor waits on the bus for either acknowledgement or Not Ready or Bus Error or Retry conditions.

The other PMIU module's spy logic monitors the system bus for this command. If the spying PMIU is holding the block as a master, it will assert retry causing the memory module to abort processing and the PMIU module to retry its request to the other PMIU module.

The memory module interprets New Data and Purge Block requests in the same way, that is, updating the requested location with new information. The difference is however, on the System Bus where the New Data request causes cache invalidations to occur while the Purge Block is ignored by the spying modules. The Purge Block does not cause invalidations because invalidations for that block already occurred before when the module purging the block requested the master using Private Read request.

PURGE BLOCK (PURGE)

Source: CHAMP processor (with NST cache) and PMIU modules.

Dest: Memory modules

Type: Used by PMIU in both ST and NST environments, used by processor modules in NST environment only, memory type, request only (one way transfer)

Cycles: Transmit Phase: 5

Receive Phase: 0

Processor: The CHAMP based processor with an NST cache issues this command on the bus to return a master block back to main memory. The processor returns the master to make room in the cache to load another block. The 4-word block to be written in memory follows the command/address cycle. After transmitting the request, the requestor waits on the bus for either Acknowledge or Not Ready or Bus Error or Retry conditions.

The other processor module's spy logic ignores this command.

Memory: After receiving the request, one of the memory modules containing the requested address acknowledges the request on the bus. The command causes the memory module to unconditionally write (overwrite) the master block at the requested address. If the memory module detects any uncorrectable errors while processing the request (after it has asserted acknowledge on the bus), it writes data into memory with the data corrupt bit on for the corrupted data word in the block.

PMIU: The PMIU module issues this command on the bus to return a master block back to main memory after it has acquired the block using a Private Read request. The 4-word block to be written in memory follows the command/address cycle. After transmitting the request, the requestor waits on the bus for either acknowledgement or Not Ready or Bus Error or Retry conditions.

The spying PMIU module ignores this system bus command.

READ TIME OF DATA (RTOD)

Source: IMP or CHAMP processor

Dest: PMIU modules

Type: Message type, request followed by 1 word time-of-day data return

Cycles: Transmit Phase: 1

Receive Phase: 1

Processor: The processor modules issue this message command on the bus to request time-of-day (TOD) information, maintained by the PMIU module in the system. The processor drives destination ID instead of an address, specifying which PMIU is to return the requested information. After transmitting the request, the requestor waits on the bus for either acknowledge followed by requested information (dataval) or Not Ready or Bus Error or Retry conditions. The other processor module's spy logic ignores this command. If the processor drives a non-existent destination ID or if the destination ID is corrupted, it will not receive acknowledgement from any of the PMIU's and will retry its request.

Memory: The memory modules ignore this command.

PMIU: The PMIU module does not issue this command, but instead processes the request for TOD from processor modules. After receiving the request, one of the PMIU modules, whose destination ID matches, acknowledges the request. When the PMIU has finished processing the request, it asserts dataval and transfers the TOD information to the requestor. If the PMIU module detects any errors while processing the request (after it has asserted acknowledge on the bus), it asserts the data corrupt signal along with dataval and corrupted TOD. The other PMIU whose destination does not match ignores this command.

SEND MESSAGE (SNDMSG)

Source: Processor and PMIU modules

Dest: Processor and PMIU modules

Type: Message type, request only (one way transfer)

Cycles: Transmit Phase: 5

Receive Phase: 0

Processor: The processor modules issue this message command on the bus to send messages to the PMIU modules. The processor drives a destination ID instead of an address, specifying which PMIU is to receive the message information. After transmitting the request, the processor waits on the bus for either acknowledgement or Not Ready or Bus Error or Retry conditions. If the processor drives a non-existent destination ID or if the destination ID is corrupted, it will not receive acknowledgement from any of the PMIUs and will retry its request.

A processor module that receives the message request from a PMIU, matches its destination ID and acknowledges the request. If a processor module detects any errors while processing the request (after it has asserted acknowledge on the bus), it writes or sends the information to its ultimate destination with the data corrupt bit on. The other processor module, whose destination ID does not match, ignores this command.

Memory: The memory modules ignore this command.

PMIU: The PMIU module issues this command on the bus to send messages to processor modules. The PMIU drives the destination ID instead of an address, specifying which processor is to receive the message information. After transmitting the request, the PMIU waits on the bus for either acknowledgement or Not Ready or Bus Error or Retry conditions. If the PMIU drives a non-existent destination ID or if the destination ID is corrupted, it will not receive acknowledgement from any of the processor modules and will retry its request.

PMIU modules that receive a message request from the processor, match their destination ID and acknowledge the request. If a PMIU module detects any errors while processing the request (after it has asserted acknowledge on the bus), it writes or sends the information to its ultimate destination with the data corrupt bit on. The other PMIU whose destination does not match ignores this command.

Described herein is a communication network for digital modules using a specialized set of dual system busses which are duplicates of each other and which have subdivided sets of lines for carrying different types of signal groups. The network enables fairness of arbitration to insure bus access to all requesting modules while still setting a priority for certain types of modules. Since the busses are duplicates of each other, then this redundancy permits operation on the one surviving bus should one of the busses fail.

While one preferred embodiment has been described it should be understood that other embodiments may still be encompassed by the invention as defined in the claims hereinunder. 

What is claimed is:
 1. A dual bus system operations network comprising:(a) a plurality of digital modules having means for sending commands and data as a sending requestor module or receiving commands and data as a receiving module and wherein each said module is connected to each bus means of a pair of dual system bus means, and, wherein one or more of said digital modules includes a first central processing module using a store-through cache memory and one or more of said digital modules includes a second central processing module using a non-store-through cache memory, and wherein each of said digital modules includes:(a1) means to request access to each bus means of said pair of system bus means; (b) said pair of dual system bus means including:(b1) a first system bus means using a synchronous transmission protocol compatible for both said store-through and non-store-through central processing modules; (b2) a second system bus means replicating said first system bus means and using said compatible synchronous transmission protocol; (b3) connection means for connecting each of said digital modules to each of said first and second system bus means; (c) said first central processing module including a first processor unit and a first store-through cache memory connected to said connection means; (d) said second central processing module including a second processor unit and a second non-store-through cache memory connected to said connection means; (e) wherein said plurality of digital modules further includes:(e1) a plurality of interface modules for interfacing said pair of system bus means to peripheral units; (e2) a memory module for holding data accessible by said first and second processor units; (f) maintenance processor means for configuring said network for exclusive use of said one or more second processing modules having said second Non-Store-Through (NST) cache memories.
 2. The network of claim 1 which includes:(a) means to limit the time period that a sending requestor module may retry its transmission.
 3. The network of claim 1 which includes:(a) means to arbitrate the priority given to bus access requestor modules and to grant bus access on either available bus to the selected requestor module in one clock cycle.
 4. The network of claim 1 which includes:(a) means to grant priority of access to said interface modules request for access to a memory module over a simultaneously occurring request for access to said memory module by said first or second processor unit; (b) means to insure that no one particular requesting module will be denied bus access during a selected period of time.
 5. The dual bus network of claim 1 which includes:(a) means for sensing when said first or second system bus means is inoperative; (b) means for disabling bus access to said sending requestor modules seeking to access said inoperative system bus means.
 6. The network of claim 1 wherein each of said first and second system bus means includes:(a) a first plurality of lines for conveying addresses, commands and data between a sending module and receiving module; (b) a second plurality of lines for conveying arbitration signals to establish priority of access for modules requesting use of said system busses; (c) a third plurality of lines for conveying control signals between sending and receiving modules: (d) a fourth plurality of lines for conveying status signals between sending and receiving modules; (e) a fifth set of lines for conveying synchronous clock signals from a clock module to each of said digital modules.
 7. The network of claim 2 which includes:(a) arbiter means connected to each of said first and second system bus means and including:(a1) means to equitably distribute priority of bus access to each of said digital modules.
 8. The network of claim 6 wherein each of said digital modules includes:(a) means to transmit a command or data on one of said system bus means while simultaneously receiving a command or data on the other one of said system bus means.
 9. The network of claim 6 which includes:(a) means to execute a bus transaction for conveying data on said dual system bus means, said bus transaction including:(a1) an Arbitration period to permit access to each one of said first and second system bus means and granting bus usage to the bus means that is available for use; (a2) a Transmit period for sending a command and address only, or for sending a command and address followed by data; (a3) a Receive period for return of data to a prior sending requestor module.
 10. The network of claim 9 wherein said bus transaction further includes:(a) means to send message commands using a destination identification field in place of said address.
 11. The network of claim 9 which includes:(a) means, for said receiving module, to transmit a Not Ready Signal to said sending requestor module; (b) means for said sending requestor module to wait until said Receive period, to retransmit its request to said receiving module.
 12. The network of claim 10 wherein said transmit period for command and address is executed in one clock cycle.
 13. In a network utilizing a dual system bus means connected to main memory and using a common protocol compatible for data transfers between processor modules with Store-Through cache memory units and processor modules with Non-Store-Through cache memory units which intercommunicate with a plurality of interbus processor interface modules using Non-Store-Through cache memory units, a system for data transfers between said processor modules and interbus processor interface modules, each of which can operate independently as a sending module and a receiving module, said system comprising:(a) means to configure said network so that a first bus is dedicated to processor modules having Store-Through (ST) cache memory units and the second bus is dedicated to said processor modules having Non-Store-Through cache memory units; (b) means to configure said network so that both first and second busses are dedicated to said processor modules having Non-Store-Through (NST) cache memory units; (c) a plurality of digital modules connected to said dual system bus means using a common data transfer protocol for each digital module, said digital modules including:(c1) first and second processor modules each having a Non-Store-Through (NST) cache memory unit; (c2) first and second interbus-processor message interface unit modules; (c3) main memory module means; (d) said dual bus means including first and second busses each of which includes:(d1) an address/command/data line; (d2) a control line for enabling/disabling said digital modules; (d3) a status line for conveying information on the status of each digital module; (d4) an arbitration line to grant/deny bus access to a requesting module; (d5) a common clock line for synchronous clock signals; (e) common data transfer protocol means for regulating data transfers between sending and receiving digital modules; (f) spy logic means in each of said first and second processor modules and said interbus processor message interface unit modules for detecting data writes occurring in any of said processor modules NST cache memory units and in any of said interbus-processor message interface unit modules; (g) means to invalidate in each module, any existing copies of data having an address that had been overwritten.
 14. The system of claim 13 wherein said first and second processor modules and said first and second interbus-processor message interface unit modules, each include:(a) means to return (PURGE), a master block of data being held, back to said main memory module; (b) means, in said main memory module, to overwrite said master block of data at the address designated by said sending module.
 15. The system of claim 13 wherein each of said first and second interbus-processor message interface unit modules include:(a) means to transmit a new data overwrite command to an address in said main memory module, said new data being originated from a peripheral device; (b) means, in said interbus-processor message interface unit module, to receive an acknowledgment signal from said main memory module. (c) means to transmit said new data to said main memory module for overwriting.
 16. In a network utilizing a dual system bus means with first and second busses which are connected to main memory and which use a common protocol compatible for data transfers between processor modules with Store-Through cache memory units and processor modules with Non-Store-Through cache memory units which intercommunicate with a plurality of interbus processor interface modules using Non-Store-Through cache memory units, a method for data transfers between said processor modules and interbus processor interface modules, each of which can operate independently as a sending module and a receiving module, comprising the steps of:(a) requesting bus access by a sending module, to any one of said dual system bus means; (b) arbitrating when said sending module will be given access to one bus of said dual system bus means based on a rotating priority access algorithm which insures fairness of access to each module; (c) transmitting, by said sending module, when bus access is granted, a command, destination address, and data words to a target receiver module; (d) checking said bus to sense if a Not Ready signal or Bus Error signal appears; (e) replying, by said target receiver module, with an acknowledgment to said sending module, if no Not Ready or Bus Error signal appears; (f) placing, by said sending module, of a Busy signal on said bus to prevent bus access by any other modules seeking bus access; (g) returning, by said receiver module, to said sending module, data words requested by said sending module; (h) releasing said bus, by said sending module, for use by another sending module; (i) configuring said network so that said first bus in said dual system bus means is dedicated to processor modules having Store Through cache memory units and said second bus is dedicated to processor modules using Non-Store-Through cache memory units; (j) configuring said network so that said first and second busses of said dual system bus means are dedicated only for use of processor modules having Non-Store-Through cache memory units.
 17. The method of claim 16 wherein step (a) includes:(a-1) Retrying for bus access, by said sending module, when said one bus is indicating a Not Ready signal, or a Busy signal.
 18. The method of claim 16 wherein said step (j) includes:(j-1) spying, on said dual system bus means, by said processor modules with Non-Store-Through cache memory units and by said interbus-processor interface units to detect a Private Read Command by a sending module to said main memory and to said other processors Non-Store-Through cache memory units; (j-2) transmitting an acknowledge signal, from one of said other processors cache memory unit indicating it has the requested data for said sending module; (j-3) aborting a response by said main memory when it senses the acknowledgment signal sent by said other processors' cache memory unit.
 19. The method of claim 18 wherein step (j-1) includes:(j-1a) requesting by said interbus processor interface unit, a copy of a block of data from: (i) a processor's Non-Store-Through cache memory unit and from: (ii) said main memory; (j-1b) spying on said dual system bus means to identify an acknowledgment signal indicating that said main memory has the requested data; (j-1c) receiving the requested data from said main memory, by said interbus processor interface unit. 